With the ever-increasing need to increase the data areal density in hard disk drives, the magneto-resistive (MR) sensor that is used as the read-back element is increasingly required to have better spatial resolution while continuing to maintain a reasonable signal-to-noise (SNR) ratio. FIG. 1 shows the structure of a typical state of the art current-perpendicular-to-plane (CPP) MR read head. This usually includes a CPP-GMR (giant-magneto-resistive) or TMR (tunneling-magneto-resistive) head, which are the two main MR sensor structures in use in state-of-the-art hard disk drives.
The CPP MR head shown in FIG. 1 has top and bottom reader shields, 11 and 12. Hard bias (HB) magnets 13, at the sides and MR sensor stack 14, are located between the reader shields. HB 13 has longitudinal magnetization 15 and provides a biasing field for sensor stack 14 thereby orienting magnetization 16 of the free layer within the stack in the longitudinal direction. In today's hard drive, the magnetic head's flying height, above the disk storage medium layer, is already less than 10 nm and is approaching the regime of less than 5 nm, so the freedom to decrease the fly height further in order to increase the reader spatial resolution is reaching its limit. Thus, contemporary attempts to increase reader spatial resolution focus on reducing the read head's reader-shield-spacing (RSS), (designated 17 in FIG. 1).
To reduce RSS, both the thickness of MR sensor stack 14 and that of HB layer 13 need to decrease. However, thinning down the HB layer also implies a smaller HB pinning field at the free layer (FL) edges that will further degrade the SNR of the sensor. Additionally, a CPP MR sensor includes a multi-layer structure in proximity to the free layer which provides a reference direction for free layer magnetization rotation during signal read-back. Further decrease of sensor stack thickness will be limited by this reference multi-layer structure, which will become unstable and produce noise at low thicknesses. Thus, for the thinner RSS design to work well, a thin stack MR sensor without the reference structure and without the strong HB field is needed.
The prior art [2][3] has suggested the possibility of limiting the sensor to three layers—two magnetically free layers positioned on opposite sides of a non-magnetic layer. The two magnetic layers should be essentially identical, having similar magnetic moments, thicknesses and dimensions. An antiferromagnetic (AFM) coupling field exists between the magnetic layers either through magnetostatic interaction or by AFM type coupling through the non-magnetic layer [4][5].
Thus, in the absence of an external field, the magnetizations of the two layers will be anti-parallel to each other. However, for this type of tri-layer MR sensor to function at its maximum sensitivity, the two magnetizations need to be oriented nearly orthogonal to each other at zero applied state. An external permanent magnet may be used to bias the two magnetizations to around 45 degree relative to the air-bearing surface (ABS). In this way, the relative angle between the two magnetizations is close to 90 degree, i.e. the maximum sensitivity position.
The prior art is, however, silent as to how such tri-layer sensors will perform when their relative physical dimensions are varied or even if such variations could play a significant role in the sensor's applicability to actual magnetic read heads. The present invention explores some of these possibilities.    [1] Y. Zhou, “Thermally Excited Low Frequency Magnetic Noise in CPP structure MR heads,” IEEE Trans. Magn., vol. 43, pp. 2187, 2007    [2] S. Mao, et al, “Differential CPP reader for perpendicular magnetic recording,” US 2005/0105219 A1 (2005)    [3] C. Hou, et al, “Biasing for tri-layer magnetoresistive sensors,” US 2005/0088789 A1 (2005)    [4] W. F. Egelhoff Jr., and M. K. Kief, “Antiferromagnetic coupling in Fe/Cu/Fe and Co/Cu/Co multilayers on Cu(111),” Phys. Rev., B vol. 45, p.p. 7795, (1992)    [5] S. S. P. Parkin, R. Bhadra, and K. P. Roche, “Oscillatory magnetic exchange coupling through thin copper layers,” Phys. Rev. Lett., vol. 66, p.p. 2152, (1991)    [6] J. Faure-Vincent et al., “Antiferromagnetic coupling by spin polarized tunneling” J.A.P. vol. 93, p. 7519 (2003)A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 7,177,122 (Hou et al), U.S. Patent Applications 2006/0002035 (Gao et al), 2005/0105219 (Mao et al), and 2005/0088789 (Hou et al) all teach the tri-layer MR sensor, but do not mention the importance of the aspect ratio of strip height to track width or ferromagnetic coupling.
In U.S. Pat. No. 7,160,572, Fujikata et al. discuss reducing surface roughness to improve the quality of the MR sensor while U.S. Patent Application 2005/0118458 (Slaughter et al) teaches reducing surface roughness to reduce orange peel coupling.